Naphthenic base oil has been base oil that has a viscosity index of 85 or less and in which at least 30% of the carbon bonds of the base oil are of a naphthenic type according to ASTM D-2140.
Recently, naphthenic base oil is widely used in various industrial fields for a variety of purposes, including transformer oil, insulation oil, refrigerator oil, oil for processing rubber and plastic, fundamental material of print ink or grease, and base oil for metal processing oil.
Conventional methods of manufacturing naphthenic base oil are mainly conducted in such a manner that naphthenic crude oil having high naphthene content (naphthene content: 30-40%), serving as a feedstock, is passed through a vacuum distillation unit to thus separate a paraffinic component and then through extraction and/or hydrogenation units to thus separate an aromatic component and/or convert it into naphthene, after which impurities are removed.
However, the conventional methods are problematic in that the essential use of the naphthenic crude oil having high naphthene content as the feedstock encounters a limitation in the supply thereof, and furthermore, the extraction procedure for extracting the aromatic component must be conducted, undesirably lowering the total product yield and deteriorating the quality of the product.
International Patent No. WO 2004/094565 discloses a method of manufacturing naphthenic base oil by subjecting a mixture composed of effluents of various process units, serving as a feedstock, to hydrofining to thus obtain oil fractions, which are then stripped to separate only an oil fraction having a boiling point within a predetermined range, and then dewaxing the separated oil fraction. However, the above method is disadvantageous because, among effluents of the hydrofining unit, only a middle oil fraction, excluding a light oil fraction and a heavy bottom oil fraction, is used to produce the naphthenic base oil, undesirably lowering the total product yield. Further, because the removing of impurities during the hydrofining process is not sufficiently performed, sulfur is contained in a large amount in the middle oil fraction separated through stripping, remarkably reducing the activity and selectivity of a catalyst used in a downstream dewaxing unit.
In addition, methods of increasing the total process yield are required.